1. Field of the Invention
The invention relates to a mixer arrangement and, in particular but not exclusively, to a mixer arrangement for use in a base station or mobile station or similar entity in a wireless telecommunications network.
2. Description of the Related Art
An area covered by a cellular telecommunications network is divided into a plurality of cells. Each of the cells has a base station arranged to transmit signals to and receive signals from mobile stations in the cell associated with the respective base station. Mobile stations will be in active communication with the base station associated with the cell in which the mobile station is located.
Both mobile stations and base stations take signals which are at a base band frequency and up convert them to a radio frequency for transmission. The base band frequency signals are either directly converted to the radio frequency or are converted via one or more intermediate frequencies. In order to up convert the base band frequency to the radio frequency directly or via the intermediate frequency or frequencies, a mixer is used. The mixer receives one input from the signal to be up converted and a second input which includes a frequency component. The second input to the mixer is generated by a local oscillator. The mixer mixes the two inputs and the resulting signal output by the mixer will have the signal information contained in the first input and will be at a frequency which is typically the sum of the input frequencies. For example, if a signal which is to be up converted has a frequency A (first input) and the signal with which it is to be mixed has a frequency B (second input), the mixer will output a frequency of A+B (or A−B in some implementations).
However, mixers which are used for this purpose have the problem that they allow the input signals to feed through the mixer. This means that the mixer will output the A+B frequency signals but also the signal at frequency A and the signal at frequency B. This can be a problem in that the frequency B is generally much greater than frequency A so that B is relatively close to the frequency A+B. Accordingly, in order to remove the unwanted B frequency, other devices have used complicated filtering solutions. For example, surface acoustic wave (SAW) filtering may be used which is both expensive and complicated to implement.
If the unwanted signal at frequency B is not filtered, this can lead to more harmful intermodulation products that can fall within the band of the wanted output. This produces loss of performance in the transmitter as the spectral purity is degraded. The unwanted local oscillator amplitude can drive some of the following power amplifier stages into compression which in turn produces more unwanted non linearity problems.
New standards are being proposed and implemented which involve using wide band multi carrier systems. It has been found that the problem of the unwanted frequency feeding through the mixer has a significant impact on such systems and can limit their performance.